679. The mesentery is a broad membrane composed of two layers of peritoneum. Between these two layers, at one extremity of the duplicature, is placed the intestines, while the other extremity is attached to the spinal column. The mesentery being much shorter than the intestines, the intestines are gathered or puckered upon the membrane, by which beautiful mechanical contrivance they are held in firm and close contact with each other, yet their convolutions cannot be entangled, nor can they be shaken from their place by the sudden and often violent movements of the body. It sometimes happens, in consequence of disease, that the convolutions of the intestines are glued together by the effusion of lymph, and then the most trifling causes are capable of producing the severest symptoms of obstruction in the bowels. 680. The internal surface of the small intestines is distinguished, 1. By the number of the mucous glands, which may be seen by a magnifying glass to consist partly of a prodigious number of the minutest follicles, not collected in groups, but equally scattered throughout; and partly of glands of a larger dimension, disposed in groups at particular parts of the canal. 2. By the increase in the number and size of the villi, of which there are about four thousand to the surface of a square inch. Like those of the stomach, the villi of the small intestine are composed of arteries, veins, nerves, and mucous ducts; but to the villi of the small intestine, in length about one-fourth of a line, there is added a new vessel, the absorbent of the chyle, the lacteal (figs. 175 and 176), so named from the milk-like chylous fluid which it contains. 3. By the great extension of the mucous coat obtained by the disposition of the membrane into the folds called valvulæ conniventes (fig. CLXXIII.). These folds, which rarely extend through the whole circle of the intestine, are often joined by communicating folds (fig. CLXXIII.). The folds are broadest in the middle, and narrowest at the extremities (fig. CLXXIII.). In general, they are about a line and a half broad. One edge of the fold is loose, but the other is fixed to the intestine (fig. CLXXIII.). The office of these folds is, first, without increasing space, to extend surface for the distribution of the villi; and, secondly, to retard the flow of the chyle, by opposing to its descent valves so constructed and disposed as, without arresting its progress, to moderate and regulate its course, in order that time may be allowed for its absorption. [Illustration: Fig. CLXXIII. Internal view of a portion of the jejunum, showing the arrangement of the mucous membrane into valvulæ conniventes.] [Illustration: Fig. CLXXIV.—_View of the Outer Coats of the Small Intestine._ 1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—_a._ longitudinal fibres. _b._ Circular fibres.]